Conventional coating thickness measuring instruments comprise an electromagnetic probe. A tip of the probe is placed in contact with the surface of a coating on a substrate. The instrument determines the distance between the probe tip and the substrate, and thereby measures the thickness of the coating.
To enable accurate measurements to be made it is necessary for an instrument to be calibrated prior to use. This is achieved by placing calibration foils of various known thicknesses onto an uncoated region of a substrate on which coating thickness is to be measured, or a sample of an equivalent substrate, and placing the probe onto the foils and taking readings. Alternatively measurements of test coatings of known thickness may be made. The resulting information is used to generate an appropriate calibration curve to calibrate subsequent measurements of coating thickness taken on the substrate or an equivalent substrate. The calibration curve is stored by the instrument as calibration data.
To ensure accuracy it is necessary to recalibrate the instrument for each substrate or type of substrate on which measurements are to be made. Changes in characteristics of a substrate can have a significant effect on measurements taken by an instrument. The need to calibrate and recalibrate can be inconvenient and time consuming. In some circumstances an appropriate sample of an uncoated substrate is not available to enable the instrument to be calibrated. The present invention addresses these problems.
It has been found that readings taken by a coating thickness measuring instrument over a substrate are significantly affected by the profile (surface roughness) of the substrate.